As is well known in the art of electrostatic copiers, conventional technology of the type herein referred to involves uniformly charging a photoconductive insulating surface by moving the insulating surface past one or more d.c. corona discharge devices which are connected to a high voltage source and which include one or more thin wires to impart a uniform charge to the surface. This charge is thereafter selectively dissipated by exposure of the surface to a light pattern. The resulting electrostatically charged latent pattern or image is thereafter developed by application of an electroscopic material called toner through electrostatic attraction to form a visible image of toner particles corresponding to the electrostatic pattern. This pattern is subsequently transferred to paper or other medium on which the images are to be imprinted. At least one other corona device is conventionally used to effect transfer of the electrostatic image to the paper. This transfer may be effected by application of a charge to the back of paper. This charge is opposite to that of the toner particles and greater than that attracting the particles to the photoconductive medium so that a sufficient number of particles are attracted to the paper to produce a clear, sharp image.
An important problem in the use of such printers arises because of the need for maintaining an acceptable level of print quality over prolonged periods of time. This problem is particularly acute in output printers wherein an exceptionally large number of pages are reproduced. Quality of a level comparable to that produced by conventional electric typewriters is expected. For such machines to be of wide spread utility commercially the quality of the finished product must be maintained over relatively long periods of time without too many service calls.
One problem arises because of the tendency of toner particles to be attracted to the face of the cathode ray tube used for the formation of characters. A certain amount of toner particles is always in the environs of the machine and a particularly large amount may be present following paper jams or similar malfunctions.
A further problem is the gradual degradation of the photoreceptor material itself. Typical cadmium sulfide photoreceptors have a service life of approximately 25,000 copies. When used as an output printer 25,000 copies is not a satisfactory service life.